This application is a 35 USC 371 application of PCT/DE 99/02199 filed on Jul. 15, 1999.
1. Field of the Invention
The invention relates to an electromechanical wheel brake device which is intended in particular for use in a motor vehicle.
2. Description of the Prior Art
One electromechanical wheel brake device is known from German Patent Disclosure DE 38 40 685 A1. This known wheel brake device has an electric motor and three series-connected planetary gears, the first of which can be driven by the electric motor. The third planetary gear actuates a brake actuating device of the wheel brake device, and the brake actuating device of the known wheel brake device has a double cam that is rotatable with the third planetary gear and that presses two brake jaws (friction brake linings) against a brake drum (brake body) of the wheel brake device, embodied as a drum brake. One of the planetary gears, preferably the first one, has two switchable couplings, with which, selectively, a ring gear of the planetary gear can be locked or a ring gear of the planetary gear can be connected to the ring gear in a manner fixed against relative rotation. In the case of the ring gear rotationally fixed to the ring gear, one planetary gear is bridged; its gear ratio is 1:1. In this state, only the two outer planetary gears effect a speed reduction, and as a result the brake actuating device is actuable fast, but with low moment or low force. In this state, an air gap between the friction brake linings and the brake body is overcome. After reversing the two couplings, the planetary gear equipped with the couplings executes a speed reduction as well; the overall result is a greater speed reduction over all three planetary gears. In this state, the brake actuating device is indeed actuated more slowly, but with greater force or greater moment. This situation serves to press the friction brake linings, already contacting the brake body, against the brake body with great force and as a result to attain a high brake force and a high braking moment.
The wheel brake device of the invention has two planetary gears and two switchable couplings. One of the two switchable couplings, in one switching position, stops a sun wheel of the second planetary gear, and in another switching position, it connects the sun wheels of the two planetary gears to one another in a manner fixed against relative rotation. The second coupling, in one switching position, stops a ring gear of the second planetary gear, while in another switching position it connects the ring gears of the two planetary gears to one another in a manner fixed against relative rotation. The wheel brake device of the invention makes four switching states of its two planetary gears possible, namely a fast gear and a slow gear for tightening the wheel brake device, a fast gear with a reversal of the direction of rotation for releasing the wheel brake device, and a locked position, in which the two planetary gears cannot retate, so that a braking force exerted is also maintained while the electric motor is without current.
The wheel brake device of the invention has the advantage that an air gap between the friction brake lining and the brake body is rapidly overcome with it, and then, by a transition to a higher gear ratio, a high contact pressure of the friction brake lning against the brake body can be attained. Another advantage is that with the same direction of rotation of the electric motor, the contact pressure of the friction brake lining against the brake body can be reduced, and the wheel brake device can be released entirely; accordingly, upon release of the wheel brake device the electric motor is operated in the same direction of rotation as in tightening of the brakes. Hence no reversal of the direction of rotation and no reversing mode of the electric motor are necessary. When operating the wheel brake device, the electric motor is merely turned on and off, and a pwer switches suffices for this purpose. Conversely, to reverse the direction of rotation and no reversing mode of the electric motor are necessary. When operating the wheel brake device, the electric motor is merely turned on and off, and a power switch suffices for this purpose. Conversely, to reverse the direction of rotation would require a full bridge with four power switches. Another advantage of the invention is that the restoration of the wheel brake device takes place at high speed, so that the wheel brake device takes place at high speed, so that the wheel brake device is released rapidly. In addition, the wheel brake device of the invention has the advantage of improved dynamics, since in a rapid alternation of tightening and releasing, or from increasing the contact pressure of the friction brake lining against the brake body to reducing the contact pressure and vice versa, the direction of rotation of the electric motor is not reversed counter to its moment of inertia, but rather is maintained unchanged, so that in a rapid alternation, the moment of inertia of the electric motor can even be utilized. This makes the wheel brake device of the invention especially well suited to anti-lock, traction control and/or electronic stability control, which necessitate a rapid alternation between tightening and release, or between increasing and reducing the braking force.
Preferably, the two switchable couplings are embodied as electromagnetic couplings, which in a basic position without current, stop the sun wheel and the ring gear of the second planetary gear. In this way, the wheel brake device is stopped, so that an imposed contact pressure of the friction brake lining against the brake body is maintained, even when the electric motor has no current. As a result, the wheel brake device can be used as a parking brake.
For converting the rotary motion of the second planetary gear into a translational motion for pressing the friction brake lining against the brake body, in one feature of the invention the actuating device of the wheel brake device has a screw thread, which is preferably embodied as a threaded roller gear for the sake of improved efficiency and low friction.